The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a method of and apparatus for controlling a chemical mechanical polisher.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, plaranization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened or soft surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-active agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
One reoccurring problem in CMP is instability in the polishing rate. In some polishing operations, the polishing rate tends to drift over time. As a result, it becomes more difficult to control endpointing and to polish each substrate by the same amount. This tends to result in dishing and erosion during metal polishing. Other reoccurring problems in CMP include temperature drift and system vibrations.
In one aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a polishing surface, a carrier head to press a substrate against the polishing surface with a controllable pressure, a motor to generate relative motion between the polishing surface and the carrier head at a velocity, and a controller configured to vary at least one of the pressure and velocity in response to a signal that depends on the friction between the substrate and the polishing surface to maintain a constant torque, frictional force, or coefficient of friction.
Implementations of the invention may include one or more of the following features. The controller may be configured to vary the pressure to maintain a constant torque, to vary the pressure to maintain a constant friction, to vary the pressure to maintain a constant frictional coefficient, to vary the velocity to maintain a constant torque, to vary the velocity to maintain a constant friction, to vary the velocity to maintain a constant frictional coefficient, to vary the velocity and the pressure to maintain a constant torque, to vary the velocity and the pressure to maintain a constant friction, or to vary the velocity and the pressure to maintain a constant frictional coefficient.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus that has a polishing surface, a carrier head to press a substrate against the polishing surface with a controllable pressure, and a pressure controller to control the pressure applied by the carrier head in response to a friction between the substrate and the polishing surface to maintain a substantially constant polishing rate.
Implementations of the invention may include one or more of the following features. The polishing surface may include a fixed abrasive polishing material. A motor may create relative motion between the polishing surface and the substrate. The pressure controller may comprise a digital computer configured to receive a motor signal representing a current in the motor to create relative motion between the polishing surface and the substrate, and to derive a carrier head pressure control signal by subtracting a threshold value from the motor signal. The digital computer may be configured to amplify or attenuate the difference between the threshold and the motor signal to determine the carrier head pressure control signal. The digital computer may be configured to smooth the carrier head pressure control signal. The motor signal may be a carrier head control signal, a platen control signal, or a motor current signal. The polishing surface may be placed on a rotatable platen and the motor may rotate the platen. The motor may rotate the carrier head.
In another aspect, the invention is directed to a method of chemical mechanical polishing. In the method, a substrate is pressed against a polishing surface with a controllable pressure, relative motion is caused between the polishing surface and the substrate at a velocity, and at least one of the pressure and velocity is controlled in response to a signal that depends on the friction between the substrate and the polishing surface to maintain a constant torque, frictional force, or coefficient of friction.
Potential advantages of the invention include zero or more of the following. A uniform frictional force may be maintained between the substrate and the polishing pad, thereby reducing fluctuations in the polishing rate. A uniform frictional force may be maintained despite variations in the pattern density on the substrate, physical properties of the polishing pad, polishing pad degradation, and changes in temperature at the pad-substrate interface. In addition, by improving the uniformity of friction, vibrations in the polishing machine and drift of the substrate temperature may be reduced. Moreover, dishing and erosion in the substrate can be reduced.
Other features, objects, and advantages of the invention will be apparent from the following description, which includes the drawings and claims.